It is known that 1-(2-deoxy-2-fluoro-4-thio-β-D-arabinofuranosyl)cytosine (henceforth also referred to as “Compound A”) has superior antitumor activity, and is useful as a therapeutic agent for tumors (Patent document 1). It is also known that Compound A shows potent antitumor activity even when it is orally administered to mice (Non-patent documents 1 and 2).